Nitrogen vacancy (N-V) centers in diamond are promising systems for use in quantum information processing. An N-V vacancy center is a point defect in a diamond crystal and corresponds to an adjacent pair of lattice sites including a nitrogen atom instead of a carbon atom at one lattice site and a vacancy, i.e., no atom, at an adjacent lattice site. N-V centers have localized electron spin states that can be electromagnetically manipulated, and state transitions in an N-V center produce photons with a sharp resonance frequency. An individual N-V center can be viewed as a basic quantum system for a quantum bit (sometimes referred to as a qubit). The qubit associated with an N-V center can be electromagnetically manipulated and remotely affected in a quantum information processor or other device using optical channels that interact with the N-V center. A quantum information system would normally contain multiple qubit devices (e.g., separate N-V centers) to perform desired quantum information processes.
A problem for N-V centers in diamond and for other solid-state quantum systems that interact with light is the difficulty of fabricating a set of such quantum systems that have identical optical couplings. For example, an interaction of two separated quantum systems to create an entangled state of the two devices might require that the quantum devices efficiently couple to a specific optical channel (e.g., to light with a specific frequency) employed in a quantum information processing system. Microcavities have been used to resonantly enhance the coherent part of the interactions of N-V centers with desired optical channels. However, fabricating many cavities that all have exactly the same resonance frequency, to within a fraction of a line or resonance width, as may be needed in some quantum information systems, can be difficult.
Another problem for quantum information systems that use light interactions is production of the quantum devices that have nearly the same optical transition frequency, which is necessary or desirable for consistent interactions with the optical channels employed in a quantum information system. However, random impurities and strain in a solid state quantum system can cause variation in the energies of the quantum states, resulting in differences in the transition frequencies of separate devices in the system. For example, N-V centers produced in a diamond lattice using current fabrication techniques have a typical variation in their transition frequencies of about 10 GHz in good material to more than 1000 GHz in a material with heavy damage or strain.
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